It is nowwell-established that pachytene mapping of the human chromosome complement is practical. The broad objectives of the proposed research include: (1) continuing, with the use of light microscopy, the mapping of human chromosomes at pachytene in the male, and if possible to initiate such mapping in the female, (2) amplifying these observations with others obtained by use of the electron microscope, (3) correlating data from pachyten mapping with those obtainable from differential staining of metaphase chromosomes, (4) establishing the approximate limits of normal morphological variibility of the chromosome complement, (5) detecting departures beyond these limits and (6) recognizing the existence of correlations between such departures and phenotypic abnormalities, including clinical disorders. Continuing efforts will be made to improve preparative methods and to apply quantitative procedures such as microspectrophotometry to human pachytene chromosomes.